Photoelectrochemical Reduction Products of Carbon Dioxide at Metal Coated p-GaP Photocathodes in Non-Aqueous Electrolytes.

Photoelectrochemical reduction of CO2 was carried out using metal-coated p-GaP photocathodes in non-aqueous electrolytes prepared from tetraalkylammonium salts and propylene carbonate as an aprotic solvent. In non-aqueous electrolytes, the coating of Au, In and Pb increased the cathodic photocurrents and the stability of the electrodes, while Zn coating did not show such significant effects. Photoelectrochemical reduction products at a bare p-GaP in non-aqueous electrolytes were (COOH)2, HCOOH, CO and H2, and the faradaic efficiency for CO formation became 50%, in contrast to that in the aqueous electrolytes where it was only a few %. The Photoelectrochemical reduction products at metal-coated p-GaP photocathodes depended greatly on the catalytic properties of the coated metal, i.e. the catalytic effect on the electrochemical reduction of CO2. By Pb coating, the faradaic efficiency for (COOH)2 formation became ca. 50%, and by Au, In or Zn coating, that for CO became almost 100%. The water content in non-aqueous electrolytes affected the faradaic efficiency of each product significantly.